The uptake of serum-bound 57Co-B12 by freshly aspirated human bone marrow cells is an active, calcium-dependent process requiring the presence of cellular respiration, oxidative phosphorylation and free sulphydryl groups. In contrast, serum-mediated uptake of 57Co-B12 by reticulocytes and erythrocytes has been reported to be an energy-independent, surface-adsorption phenomenon. Bone marrow cells also take up 57Co-B12 in the absence of serum. Such uptake is dependent on cellular respiration but not on calcium ions or free sulphydryl groups. The average value for the uptake of serum-bound 57Co-B12 by vitamin B12-deficient bone marrow cells was 2.7 times greater than that for normoblastic marrow cells.